Functional films composed of ferroelectric material or piezoelectric material are used in some electronic devices such as film capacitors, ferroelectric nonvolatile memories, film bulk acoustic wave resonators and the like. A film bulk acoustic wave resonator, for instance, is configured by sandwiching a functional film made of a piezoelectric material between an upper conductive film and a lower conductive film. The film bulk acoustic wave resonator operates as a high-frequency filter when a high-frequency signal is applied across the two conductors.
In order to achieve good device characteristics in such an electronic device, it is necessary to enhance the crystallinity of the functional film. In order to enhance it, it is important not only to optimize the functional film-forming method and conditions but also to improve the crystallinity of the lower conductive film serving as the base for the functional film. This is because the crystallinity of the lower conductive film serving as the base markedly affects the crystallinity of the functional film. A method of forming a lower conductive film with good crystallinity is taught by, for example, Japanese Patent Application Laid Open No. 11-312801.
The material of the lower conductive film needs to be one having high lattice matching and low reactivity with respect to the functional film formed thereon. When, for example, ZnO or some other such piezoelectric material of wurtzite crystal structure is used as the functional film material, therefore, the lower conductive film serving as the base is preferably a (111)-oriented film of a metal that assumes a face-centered cubic crystal structure, such as platinum (Pt), gold (Au), iridium (Ir) or rhodium (Rh).
However, in some cases forming a lower conductive film or functional film or the like requires a high-temperature process at a temperature exceeding 600° C. Since it is necessary in such cases to form the lower conductive film or functional film on a single crystal substrate of silicon (Si) or the like or on a ceramic substrate, when growing the film, a support substrate of silicon single crystal or ceramic or the like becomes a constituent component of this type of electronic device. As such, it is often difficult to integrate such an electronic device on the same substrate as other electronic devices, limiting the product configuration.